The present invention relates to an ink film thickness control method for a printing press and, more particularly, to an ink film thickness control method for an ink supply apparatus for supplying an ink in an ink fountain to a printing plate through an ink roller group.
FIG. 10 shows the schematic arrangement of an ink supply apparatus in a web offset printing press. Referring to FIG. 10, reference numeral 1 denotes an ink fountain for storing an ink 2; 3, an ink fountain roller for supplying the ink stored in the ink fountain 1 to an ink roller group 6; 4, ink fountain keys arranged in the axial direction of the ink fountain roller 3; 5, an ink ductor roller arranged between the ink fountain roller 3 and the ink roller group 6; and 7, a printing plate mounted on a plate cylinder 20 to which the ink is supplied through the ink roller group 6.
In this ink supply apparatus, the ink 2 in the ink fountain 1 is supplied to the ink fountain roller 3 by adjusting the aperture ratios of the ink fountain keys 4. The ink supplied to the ink fountain roller 3 is supplied to the printing plate 7 on the plate cylinder 20 through the ink roller group 6 which is rotated in accordance with the feed operation of the ink ductor roller 5 in the operation of the printing press.
In the web offset printing press, when the printing plate is changed to a new printing plate 7, the aperture ratio of each ink fountain key and the rotation ratio of the ink fountain roller 3 are preset to values corresponding to the image of the printing plate 7. More specifically, the aperture ratio of each ink fountain key 4 and the rotation ratio of the ink fountain roller 3 are set to values corresponding to the image of the printing plate 7, and the ink 2 in the ink fountain 1 is supplied to the printing plate 7 through the ink roller group 6. In this case, test printing is performed before final printing to adjust the ink supply amount, thereby obtaining a satisfactory color tone. With this operation, a desired ink film thickness distribution (gradient of thickness of the ink film) is formed on the ink roller group 6.
In the conventional ink supply apparatus, however, when the printing plate is changed to the new printing plate 7, the ink film thickness distribution for the previous printing plate remains on the ink roller group 6. For this reason, the ink film thickness distribution for the previous printing plate must be gradually changed to the ink film thickness distribution for the new printing plate 7. This operation excessively requires adjustment of the ink supply amount and test printing until a satisfactory color tone is obtained, resulting in various problems including an increase in preparation time for printing, an increase in work load, waste of printing materials, a decrease in production efficiency, and an increase in cost.
Before exchange of the printing plate, the feed operation of the ink ductor roller 5 may be stopped to perform printing on blank paper, thereby nullifying the ink film thickness distribution on the ink roller group 6. With this operation, the ink roller group 6 has no ink. However, the ink film thickness distribution for the printing plate 7 must be formed on the ink roller group 6 from the beginning. In this case, a long time is required to obtain an equilibratory ink film thickness distribution, so the above-described various problems cannot be avoided. These problems are also posed when the ink roller group 6 has no ink in the initial state.